Switching direct current (DC) to DC converters are used in a variety of applications for converting power at an input voltage into power at a desired output voltage. One application in which DC-to-DC converters are commonly used is to provide power in data centers. There are a large number of data centers worldwide and their number continues to grow. The resultant energy consumption of such data centers is becoming a significant (and growing) portion of the overall worldwide energy consumption. Hence, there is a significant motivation to improve the efficiency of power converters used in data centers.
Data centers and other electronic systems have, for many years, relied upon a 12V intermediate DC bus for supplying power to a variety of circuits. Such a 12V DC bus may be widely distributed within a system and typically is down-converted to a lower voltage, e.g., 3.3V, 1.8V, 1.2V, for use by loads requiring lower voltage levels. Such down-conversion is preferably performed at the point-of-load (POL), so that the voltage level required by the load may be more efficiently regulated.
Modern power distribution systems for data centers and related are moving to an intermediate DC bus having a voltage of 40V to 60V. By using a higher intermediate DC voltage such as this, the power loss incurred in distributing the intermediate DC voltage is reduced. The intermediate DC voltage may be provided by an unregulated first conversion stage that is highly efficient. A second conversion stage is preferably located near the POL, and requires a larger step-down ratio than the case wherein the intermediate voltage is only 12V.
A common implementation of a second conversion stage, when using an intermediate DC voltage in the 40V to 60V range, uses a switched-capacitor converter based upon a Dickson charge pump topology. Such a switched-capacitor converter uses switches to transfer charge among capacitors, thereby reducing a provided intermediate input voltage, e.g., from an input of 48V to an output of 12V for a 4:1 switched-capacitor converter. Without appropriate consideration for the switch timing in such converters, losses are incurred due to hard switching, i.e., switching the switches when there is non-zero voltage across them and non-zero current flowing through them.
The above-described hard switching may be mitigated by placing an inductor in series with the capacitor at each stage of the switched-capacitor converter, thereby borrowing a design technique used in resonant (or semi-resonant) switching converters such as those based upon a center-tapped inductor. The resultant switched-capacitor converter is sometimes termed a switched tank converter (STC). The resonant tank formed by the series connection of the inductor and capacitor has an associated resonant frequency that is based upon the inductance and capacitance of these components. If the switches are switched at this resonant frequency, zero current switching (ZCS) may be achieved leading to reduced switching losses and good efficiency in the power conversion.
Matching the switching frequency of the STC to the resonant frequency of one or more resonant tanks within the STC presents practical challenges beyond the challenges associated with conventional resonant switching converters that are not based on a switched-capacitor topology. Such conventional resonant converters may characterize the reactance of the resonant tank, e.g., during a calibration stage, and set the switching frequency accordingly. Alternately or additionally, a controller of such a conventional resonant converter may measure voltage and/or current through the switches and dynamically adjust the switching frequency to match the natural resonant frequency of a resonant tank. Such techniques are generally not feasible in an STC. To achieve down-conversion rates greater than 2:1, multiple switch stages are required within the STC, each of which has its own resonant tank. Because the controller must switch the switches in each switch stage using the same frequency, it is not feasible, at least without significant control complexity, to adjust the switching frequency to match individual resonant tanks when there are multiple switch stages. Furthermore, techniques that rely upon measurement of current and/or voltage add undesirable circuit complexity and often introduce additional inefficiencies to the converter.
An STC having multiple switch stages, hence, uses multiple resonant tanks that need to have nearly identical resonant frequencies. This, in turn, requires that the inductance-capacitance product of each resonant tank be the same. Standard inductors and capacitors have fairly large tolerances, e.g., ±10%, ±20%. Furthermore, the inductance and capacitance of standard inductors and capacitors depend heavily on temperature. Therefore, use of standard inductors and capacitors typically leads to significantly different resonant frequencies across the switch stages of an STC, and these resonant frequencies may have a high variation over temperature. A controller switching all of the switches at the same frequency will typically not be able to achieve zero current switching in all of the switch stages when using standard components. This, in turn, leads to inefficiency in the power conversion of the STC. To overcome this problem, STCs may use high-precision capacitors and inductors that have a small temperature dependence. However, such components are more expensive and physically larger than standard inductors and capacitors.
Circuits and associated techniques are desired that would allow for a reduction in the size and cost of inductors and capacitors used in STCs, while achieving efficiencies associated with soft switching.